1. Field of the Invention
The present invention relates to disposal of wastes containing formaldehyde, and more particularly to a test device, method, and indicators for confirming the neutralization of formaldehyde by the calorimetric determination of excess formaldehyde neutralizer in aqueous solution.
2. Description of the Related Art
Formaldehyde is used in a wide variety of industrial and laboratory applications. Disposal of formaldehyde waste through sewer systems is inexpensive and convenient because formaldehyde is very soluble in water and is degraded by a variety of microorganisms. However, because formaldehyde is toxic, disposal of wastes containing formaldehyde requires special procedures.
The United States Environmental Protection Agency (EPA) regulations require that solutions containing formaldehyde are neutralized before being disposed into a sewer system. More specifically, the EPA requires neutralized formaldehyde solutions to be tested to verify that residual formaldehyde is below 10 ppm and that the pH is in the neutral range 6.0 to 8.0.
The safest and most practical commercial method for the neutralization of formaldehyde is by exposure of formaldehyde to sulfite ion, usually by adding sodium sulfite, sodium bisulfite, or a mixture thereof to the aqueous solution containing formaldehyde. The reactions of formaldehyde with sodium sulfite and sodium bisulfite, respectively, are the following equilibrium reactions: 
whereby sodium sulfite and sodium bisulfite each react with formaldehyde to yield sodium formaldehyde bisulfite (HOCH2NaSO3), a very stable compound in aqueous solution. The equilibrium for this reaction is shifted strongly to the right (towards the formation of sodium bisulfite formaldehyde) and the relative concentrations of sulfite ion and the free formaldehyde in the solution are very low after the reaction which forms sodium bisulfite formaldehyde.
Known methods for the determination of whether formaldehyde has been neutralized in an aqueous solution have been directed toward the direct detection or measurement of the amount of free formaldehyde in the solution, and have proven problematic for several reasons. Several prior known methods are impractical and difficult to use, such as several qualitative methods for determining formaldehyde in aqueous solutions described in Walker, J. F., Formaldehyde, 3rd ed., 483-488 (1964). For example, one such method is to measure the specific gravity and refractivity of the solution, and then estimate the formaldehyde content of the solution from those measurements using a ternary diagram, such as that described in Natta, G., Baccaredda, M., Giorn. chim. ind. applicata, 15, 273-81 (1933). This method is not useful from a practical standpoint, as it requires the measuring of both the specific gravity and refractivity of the solution, yields only an estimate of formaldehyde concentration, is cumbersome to perform in the laboratory, and is only effective for pure aqueous formaldehyde solutions, or aqueous formaldehyde solutions containing only a very small percentage of impurities.
A more accurate method of determining formaldehyde in aqueous solutions is the sodium sulfite titration method, described by Walker, pp. 486-488. While useful in the analytical laboratory for determining the concentration of formaldehyde in aqueous solution, the sodium sulfite titration method is cumbersome to perform, both in large scale and in small scale disposals, as a qualitative method of determining whether aqueous solutions of formaldehyde have been neutralized. The sodium sulfite titration method is also time-consuming to perform, and may require multiple titrations.
Other qualitative calorimetric tests for the direct determination of formaldehyde are disadvantageous because they require acidic conditions. The accuracy of these methods is questionable, because the sodium bisulfite formaldehyde product, while stable in neutral conditions, becomes unstable in acidic conditions and decomposes to form free formaldehyde and sulfurous acid. Noller, C. R., Chemistry of Organic Compounds, 2nd ed., 201-202 (1957). Not surprisingly, these types of tests do not give meaningful results.
For example, a prior known method for the determination of formaldehyde in acidic conditions involves reacting formaldehyde with 3-methyl-2-benzothiazolone hydrazone (MBTH), followed by oxidizing the resulting adduct with ferric chloride in 1.6% sulfamic acid. Hauser, T. H. and Cummins, R. L., Anal. Chem. 36, 679-681 (1964). Sulfamic acid is a strong acid that will disrupt the sodium bisulfite formaldehyde complex to yield free formaldehyde, as noted by Noller, p. 202. In addition, any excess sulfite/bisulfite present in the neutralized formaldehyde solution reduces the ferric chloride and blocks the oxidation of the MBTH/formaldehdye adduct. As a result, this method is ineffective for the determination of whether formaldehyde has been neutralized with sodium sulfite/bisulfite in an aqueous solution.
Another known method for the determination of formaldehyde in acidic conditions involves the reaction of Fuchsin, a pink triphenylmethane dye, with sulfurous acid to yield the colorless leucosulfonic acid, also known as xe2x80x9cSchiffs reagent.xe2x80x9d Schiffs reagent is unstable, and reacts with aldehydes to form a violet-purple quinoid dye, known as the Fuchsin-aldehyde reagent. Shriner, R. L., Fuson, R. C., and Curtin, D. Y., The Systematic Identification of Organic Compounds, 4th ed., 114-115 (1956). The colored Fuchsin-aldehyde reagent is not useful for the determination of formaldehyde because, as described above, the acidic conditions needed for the assay disrupt the sodium bisulfite formaldehyde complex to yield free formaldehyde. Noller, pp.201-202; Shriner et al., pp.149-150.
Still other qualitative calorimetric methods for the direct determination of formaldehyde are disadvantageous because they require alkaline conditions. Similar to acidic conditions, alkaline conditions also affect the stability of sodium bisulfite formaldehyde. For example, one test for the direct determination of formaldehyde uses 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole, supplied under the commercial name Purpald(copyright) by Sigma-Aldrich Co., Inc., (Purpald(copyright) is a registered trademark of Aldrich Chemical Co., Inc.) or a variant of this compound, as a calorimetric indicator in alkaline solution. This test is supplied by Sakura Finetek U.S.A., Inc. under the commercial name Tissue-Tek(copyright) NEUTRALEX(trademark) Aldehyde Test Kit. (NEUTRALEX(trademark) is a registered trademark of Scigen, and Tissue-Tek(copyright) is a registered trademark of Sakura Finetechnical Co., LTD). In this test, the 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole reacts with aldehydes, and is oxidized by oxygen in air to yield a purple-to-magenta-colored 6-mercapto-s-triazolo-[4,3-b]-s-tetrazine, as described in Technical Information Bulletin Number AL-145 from Aldrich Chemical Co. (citing Dickinson, R. G., Jacobsen, N. W., Chem. Commun., 1719 (1970)). As demonstrated in Example I and described below, this assay is of doubtful utility.
First, the strong alkaline conditions necessary for the 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole to react with aldehydes adversely affect the stability of sodium bisulfite formaldehyde, such that the assay result is not indicative of the actual amount of free formaldehyde present in the test solution.
In addition, as demonstrated in Example I, the Tissue-Tek(copyright) test strips indicate relatively low formaldehyde levels in standard solutions actually known to have relatively high levels of formaldehyde. It is believed that the sulfite ion in the aqueous solution blocks the oxidation of the formaldehyde/4-amino-3-hydrazino-5-mercapto-1,2,4-triazole adduct which is required to form the purple-to-magenta colored triazine.
Furthermore, in solutions with relatively low formaldehyde levels, consumption of the small amount of free formaldehyde by the colorimetric indicator causes the equilibrium between free formaldehyde and sodium bisulfite formaldehyde to shift to the left, that is, away from the formation of sodium bisulfite formaldehyde and toward the formation of more free formaldehyde. Problematically, this type of colorimetric test can consume a relatively large fraction of the free formaldehyde in a solution that contains relatively low formaldehyde levels initially, such as near the EPA requirement of 10 ppm or less. Eventually, the reversible equilibrium will allow as much of the formaldehyde in sodium bisulfite formaldehyde as is necessary to become available for reaction with the 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole, leading to observation of a false high formaldehyde level.
It is desirable to devise a test that will determine whether formaldehyde has been neutralized in an aqueous solution containing formaldehyde, where the test does not rely on a direct detection or measurement of the amount of free formaldehyde in the solution.
It is also desirable to devise a test that will enable the user to obtain a quick, reliable, and visual qualitative determination of whether formaldehyde has been neutralized in an aqueous solution containing formaldehyde.
Further, it is desirable to devise a test that is effective for use with aqueous solutions containing formaldehyde that have a pH within the substantially neutral pH range of 6.0 to 8.0 required by the EPA.
It is also desirable to devise a test that will not disturb the equilibrium between free formaldehyde and sodium bisulfte to cause the formation of free formaldehyde.
The present invention involves confirming the neutralization of formaldehyde by exposure to sulfite ions, using an indicator for detecting the presence of excess sulfite ions. The kit or test medium includes an indicator for the calorimetric determination of whether a sufficient amount of a formaldehyde neutralizer, a chemical compound which produces sulfite ions in aqueous solutions, has been added to an aqueous solution initially containing formaldehyde, wherein the solution is contacted with a test medium including a dye capable of exhibiting a color change in the presence of excess sulfite ion over formaldehyde in the solution, with the color change indicating the presence of excess sulfite ions and therefore that the formaldehyde has been substantially eliminated. The test medium may include a test strip with the dye impregnated therein or a liquid solution containing the dye.
The present invention introduces the novel approach of determining whether formaldehyde is neutralized by detecting excess formaldehyde neutralizer, rather than detecting or measuring the amount of free formaldehyde directly. An indicator dye capable of exhibiting a color change in the presence of excess formaldehyde neutralizer is used to detect whether the formaldehyde has been neutralized, where the indicator dye does not exhibit a color change when the formaldehyde neutralizer is in the presence of excess formaldehyde.
Sulfite ion, used to neutralize formaldehyde, is a reactive reducing agent. Indicator dyes or other reagents that change color upon reduction by sulfite ion at substantially neutral pH are utilized to detect the presence of excess sulfite ion. The indicator dyes exhibit a loss of color, or are xe2x80x9cbleachedxe2x80x9d by sulfite ion at substantially neutral pH. Surprisingly, the indicator dyes were not bleached by sulfite ion in the presence of a stoichiometric excess of free formaldehyde over sulfite ion. However, when the sulfite ion level was increased to give a stoichiometric excess over the free formaldehyde present the indicator dyes were bleached. Therefore, the indicator dyes are bleached only after all of the formaldehyde has been neutralized by sulfite ion, and bleaching of the dyes provides a reliable colorimetric indication that the formaldehyde has been neutralized.
In one form thereof, a kit is provided for confirming the neutralization of formaldehyde by exposure to sulfite ions in an aqueous solution, the kit including an indicator for detecting the presence of sulfite ions.
In another form thereof, a test strip is provided for determining whether a sufficient amount of a neutralizer which produces sulfite ions in aqueous solution has been added to an aqueous solution to neutralize formaldehyde initially present in the aqueous solution, the test strip including a test medium impregnated with a dye, the dye capable of exhibiting a color change upon reaction with a stoichiometric excess of sulfite ion over formaldehyde in the aqueous solution.
In another form thereof, a method is provided of confirming the neutralization of formaldehyde by exposure to sulfite ions in an aqueous solution, where the aqueous solution is exposed to an indicator that detects the presence of sulfite ions.
In another form thereof, a method is provided of substantially neutralizing formaldehyde in an aqueous solution initially containing formaldehyde by the addition of a neutralizer that introduces ion into solution, including the steps of adding an amount of the neutralizer to the aqueous solution; providing a test strip, the test strip including a dye which is reactive with sulfite ions and exhibits a color change upon reaction with sulfite ions; contacting the test strip with the solution; inspecting the test strip for a color change; and ceasing performing the previous steps when the test strip exhibits the color change.
In another form thereof, a method is provided of confirming the neutralization of formaldehyde in aqueous solution containing formaldehyde, including the steps of providing a test strip impregnated with the dye capable of exhibiting a color change in the presence of a stoiciometric excess of sulfite ion over the amount of formaldehyde in the aqueous solution; contacting the aqueous solution with the test strip; inspecting the test strip for the color change; and if the color change is not observed, adding a compound which produces sulfite ions upon dissolution to the aqueous solution and repeating the steps until the test strip exhibits the color change.
In still another form thereof, a method is provided of substantially neutralizing formaldehyde in an aqueous solution initially containing formaldehyde by the addition of a neutralizer that produces sulfite ions in a solution, including the steps of adding an amount of the neutralizer to the solution; adding to the solution a dye having an initial color, the dye capable of exhibiting a loss of the initial odor in the presence of a stoichiometric excess of sulfite ion over the amount of formaldehyde in the solution; inspecting the solution for the color loss; and repeating the first step until the solution exhibits the color loss.
In yet another form thereof, a test kit is provided for determining neutralization of formaldehyde by exposure to sulfite ions in a solution initially containing formaldehyde, including a neutralizer that produces sulfite ions in aqueous solution; and a test strip including a dye capable of exhibiting a color change in the presence of a stoichiometric excess of sulfite ions over the amount of formaldehyde in the solution.
In order to neutralize the formaldehyde in the aqueous solution, several sulfite-containing components may be used, including alkali metal sulfites and bisulfites such as sodium sulfite, sodium bisulfite, potassium sulfite, potassium bisulfite, lithium sulfite, and lithium bisulfite, or mixtures of the foregoing compounds.
Suitable indicator dyes include dyes capable of exhibiting a color change upon reaction with a stoichiometric excess of formaldehyde; dyes having a reduction potential at neutral pH of 0.064 volt or greater; dyes capable of exhibiting the color change upon reaction with a stoichiometric excess of sulfite ion at a solution pH of from about 4.4 to about 9.4; dyes selected from indophenols, indoanilines, and indamines; and dyes selected from 2,6-dichlorophenol-indophenol and thionin. Additionally, concentrations of about 0.25 mM or less of the dye may be included within the test trip, and the test strip may be formed of a bibulous material mounted on a rigid backing material such as polystryrene.
One advantage of the present invention is that it facilitates a determination of whether formaldehyde has been neutralized by the detection of an excess amount of formaldehyde neutralizer, insuring that all of the free formaldehyde in the solution is neutralized.
Another advantage of the present invention is that the test reaction consumes only a very small amount of the excess neutralizer such that the test reaction will not upset the solution equilibrium to lead to the formation of more formaldehyde.
Another advantage of the present invention is that the test is effective within the range of 6.0 to 8.0, as specified by the EPA guidelines, but the test may also effectively test for formaldehyde neutralization within a substantially wider pH range.
Yet another advantage of the present invention is that the indicator dye may be included within a test strip which is directly contacted with the test solution, making the test easier to use.
Further, a buffer may also be included within the test strip to maintain the pH of the test reaction substantially close to neutral.
Still another advantage of the present invention is that it allows the user to make a quick and reliable visual determination of formaldehyde neutralization, thereby providing a test that is practical and easy to use.
The embodiments disclosed below are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following detailed description and described in the following examples.
An indicator dye capable of exhibiting a color change in the presence of excess formaldehyde neutralizer is used to detect whether formaldehyde in an aqueous solution has been neutralized. The indicator dye does not exhibit a color change when the formaldehyde neutralizer is in the presence of excess formaldehyde.
Suitable indicator dyes include dyes which are capable of exhibiting a color change upon reaction with a stoichiometric excess of sulfite ion over formaldehyde in aqueous solution; dyes which have a reduction potential at neutral pH equal to or greater than 0.064 volt; dyes capable of reacting with an excess of sulfite ion over formaldehyde in a solution to exhibit the color change at a pH from about 4.4 to about 9.4; dyes which are selected from indophenols, indoanilines, and indamines; and dyes are selected from 2,6-dichlorophenol-indophenol and thionin.
Formaldehyde in aqueous solution may be neutralized by exposure to sulfite ion. Formaldehyde reacts rapidly with sulfite ion generated from a neutralizer, which herein refers to any sulfite-containing chemical component which produces sulfite ion upon dissolution in aqueous solution. For example, formaldehyde is neutralized effectively by sodium sulfite (3), sodium bisulfite (4), or a mixture thereof to yield sodium bisulfite formaldehyde, shown in the following net ionic equations: 
Sodium bisulfite formaldehyde is very stable in aqueous solution. However, it is not soluble in ethanol and will crystallize from aqueous solution upon the addition of ethanol to one of the following two forms containing waters of crystallization:
HOCH2NaSO3.H2O
and
(HOCH2NaSO3)2.H2Oxe2x80x83xe2x80x83(5)
Walker, p. 252. Sodium bisulfite formaldehyde is sufficiently stable in aqueous solution such that the equilibrium shown above in equations (3) and (4) is shifted greatly to the right, that is, toward the formation of sodium bisulfite formaldehyde and away from sodium sulfite/bisulfite and formaldehyde. In fact, the position of the equilibrium shown in equations (3) and (4) is farther to the right with formaldehyde than with other aldehydes and ketones. Noller, p. 202. Therefore, when sodium sulfite/bisulfite is added to an aqueous solution containing formaldehyde, the equilibrium concentration of sodium bisulfite formaldehyde will greatly exceed that of both sulfite ion and free formaldehyde.
In addition, other chemical compounds which produce sulfite ions upon dissolution, such as alkali metal sulfites, alkali metal bisulfites, or mixtures of alkali metal sulfites and alkali metal bisulfites, for example, may be used as formaldehyde neutralizers. This is because stable compounds analogous to sodium bisulfite formaldehyde may be formed by the reaction of other alkali metal sulfites/bisulfites with free formaldehyde. For example, potassium sulfite (6) or potassium bisulfite (7) may also be used as an effective formaldehyde neutralizer, reacting with free formaldehyde to form potassium bisulfite formaldehyde, similar to reactions (3) and (4) above, shown in the following net ionic equations: 
Safe neutralization of formaldehyde with sulfite ion requires a stoichiometric quantity of sulfite ion equal to that of formaldehyde, plus an excess quantity of sulfite ion to maintain the equilibrium toward sodium/potassium bisulfite formaldehyde and to keep the free formaldehyde below the 10 ppm level specified by EPA.
Sulfite ion is a reactive reducing agent that reduces certain indicator dyes such that the indicator dyes will exhibit a color change in the presence of sulfite ion. Usually, these indicator dyes either partially or totally lose their color, or are xe2x80x9cbleachedxe2x80x9d upon reduction by sulfite ion. Surprisingly, it has been found that sulfite ion will reduce the indicator dyes in the presence of free formaldehyde in aqueous solution only very slowly and therefore the indicator dyes fail to change color for a significant time period, if at all. This is due to the fact that sulfite ion will preferably react with free formaldehyde to yield the stable sodium bisulfite formaldehyde, therefore, the concentration of sulfite ion is very low in the presence of free formaldehyde such that only a very small amount of sulfite is available to reduce the indicator dyes. However, when the concentration of sulfite ion is increased to give a stoichiometric excess over the formaldehyde present, for example, by the addition of more sodium sulfite/bisulfite to the solution the indicator dyes are reduced or xe2x80x9cbleachedxe2x80x9d by the excess of sulfite ion to exhibit the color change.
The unexpected failure of sulfite ion to bleach the indicator dyes in the presence of formaldehyde was unpredictable before experiment. In addition, it was also unpredictable whether consumption of sulfite ion by the reduction of the indicator dyes would disturb the equilibrium, causing it to shift rapidly away from sodium bisulfite formaldehyde and toward the formation of more sulfite ion to cause bleaching of the dyes by sulfite ion released from sodium bisulfite formaldehyde. However, it has been found that sulfite ion bleaches the indicator dyes in the presence of free formaldehyde only very slowly, if at all, and in addition, that the equilibrium remains stable upon consumption of sulfite ion by the reduction of the indicator dyes.
Indicator dyes, for the purposes of this specification, are substances which, upon reaction with sulfite ion, exhibit an observable color change. Many indicator dyes exhibit a color change upon reduction by sulfite ion at substantially neutral pH. Generally, the reduction potential of sulfite ion indicates the capacity of bisulfite/sulfite to reduce indicator dyes present in the test solution. The reduction potential for sulfite ion under acidic and basic conditions is 0.20 volt (for H2SO3) and xe2x88x920.92 volt (for Na2SO3), respectively. Handbook of Chemistry and Physics, 52nd ed., R. C. Weast, Ed., D113 (1971). At neutral pH, the solution potential of the test solution will be between 0.20 volt and xe2x88x920.92 volt, and the sulfite ion present in the test solution will reduce indicator dyes with reduction potentials (Em7, at pH 7.0) higher than the solution potential. Therefore, indicator dyes with relatively higher reduction potentials will be reduced by sulfite ion more completely in the test solution.
The reduction potentials of a group of organic compounds are listed in Clark, W. M., Oxidation-Reduction Potentials of Organic Systems, 131-133 and 403-411 (1960). In Example II below, it is shown that the reduction potential of indigo carmine (Em7=xe2x88x920.125 volt) is too low for effective reduction by sulfite ion, whereas in Example III below, it is shown that the reduction potential of thionin (Em7=0.064 volt) is adequate for effective reduction by sulfite ion. Generally, indicator dyes with reduction potentials equal to or greater than 0.064 volt may be effectively reduced by sulfite ion. These indicator dyes include indophenols, such as those listed by Clark, p. 403-405, as well as indoanilines and indamines, such as those listed by Clark, p. 407-411.
Exemplary indicator dyes include 2,6-dichlorophenol-indophenol (xe2x80x9cDCIPxe2x80x9d) and thionin.
2,6-dichlorophenol-indophenol (xe2x80x9cDCIPxe2x80x9d) (Em7=0.217 volt) is commercially available from Sigma Chemical Co., St. Louis, Mo. and from Aldrich Chemical Co., Milwaukee, Wis.
Thionin (also referred to as xe2x80x9cLauth""s Violetxe2x80x9d) (Em7=0.064 volt) is commercially available from Aldrich Chemical Co., Milwaukee, Wis.
Indigo Carmine (also referred to as xe2x80x9cAcid Blue 74xe2x80x9d) (Em7=xe2x88x920.125 volt) is commercially available from Aldrich Chemical Co., Milwaukee, Wis.
The reduction of indigo carmine, DCIP, and thionin by sulfite/bisulfite in aqueous solution, respectively, is shown in the following equations, where indigo carmine, DCIP, and thionin are each bleached to exhibit a loss of color from blue to colorless upon reduction by sulfite/bisulfite:
(8) indigo carmine: 
(9) DCIP: 
(10) thionin 
While specific examples of dyes suitable for use with the present invention have been enumerated hereinabove, such is not to be construed as limiting the invention in any manner. Rather, one of ordinary skill in the art may identify a virtually limitless number of indicator dyes related to the exemplary indicator dyes above that may be used in accordance with the present invention.
As described above and in the following examples, the reduction of the indicator dyes by sulfite ion is pH dependent. A buffer may be impregnated within the test strip with the indicator dye to control the pH of the test reaction when the pH of the test solution varies 5 from neutral. Buffer substances having a pKa close to 7.0 at 250xc2x0 C. may be used, however, any buffer substances having a pKa in the range between about 6.0 and about 8.0 at 25xc2x0 C. may also be used. Several such suitable buffer substances are listed in Perrin, D. D., Dempsey, B., Buffers for pH and Metal Ion Control, 156-163 (1974).
For the purposes of this specification, test medium refers to any medium or carrier in which an indicator dye may be held, which may include a test strip or a liquid solution, for example. The indicator dyes described above may be impregnated within a test strip, thereby providing a test device or test medium for the easy and rapid determination of the presence of excess sulfite ion, corresponding to the neutralization of formaldehyde. An indicator dye is first prepared in a buffer solution as described above. Then, a test strip paper, such as Schleicher and Schuell 903 paper (available from Schleicher and Schuell, Inc., Keene, N.H.) is impregnated with the indicator dye/buffer solution. A bibulous material, such as filter paper, may be used as the carrier matrix to be impregnated with the indicator dye/buffer solution.
The concentration of the indicator dye in the indicator dye/buffer solution which is impregnated into the test strip should be minimized such that the reduction of the indicator dye consumes a minimal amount of sulfite ion in the test reaction. However, the indicator dye concentration must also be great enough to provide a color change in the test strip which is unambiguous and distinctly readable for most users of the test strips. In Example IV, concentrations of 0.25 mM and 0.08 mM of DCIP and Thionin, respectively, were found to be effective to produce a distinctly readable color change upon reduction by sulfite ion while consuming only a nominal amount of sulfite ion. However, it should be understood that lesser concentrations of indicator dyes may be used in test strips to obtain test strips which are functional in accordance with the present invention.
The impregnated test strip is then dried in a manner that does not adversely affect the solution impregnated within the test strip. A forced air oven may be used to dry the test strip, such as a Model 18 forced air oven manufactured by Precision Scientific of Winchester, Va. The test strip may be dried, for example, at a temperature from 120xc2x0 to 200xc2x0 F. for 3 to 30 minutes, where the drying time is dependent upon the temperature and the amount of air circulation around the test strip. If drying temperatures lower than 120xc2x0 F. are used, drying times longer than 30 minutes are typically required. After drying, the test strips may then be fabricated into test pads. The test pads are mounted onto and secured to a rigid backing material, such as polystyrene, by any suitable means, such as with a double sided tape (415 polyester tape, supplied by 3M Company, St. Paul, Minn.).
Alternatively, an indicator may be used to confirm the neutralization of formaldehyde in aqueous solution without using a test strip, as demonstrated below in Example V, where the test medium may include a liquid solution of the dye. First, a neutralizer such as sodium sulfite/bisulfite is added to the solution to neutralize the formaldehyde, and the pH of the solution is adjusted to the neutral range, if necessary. Alternatively, a buffer may be added to the solution before the addition of the neutralizer to maintain the pH in a desired range, such as the buffers described above. Then, a small amount of a solution of an indicator dye is added to the solution. Generally, the indicator dye will change the color of the solution according to the color of the indicator dye, and, if the solution remains colored, the dye has not been reduced by an excess amount of sulfite ion, indicating that the formaldehyde has not been neutralized. If the solution remains colorless after the addition of the indicator dye or becomes colorless after a certain elapsed time, the dye has been reduced by an excess amount of sulfite ion, indicating neutralization of the formaldehyde in the solution. The necessary elapsed time by which the solution must change color to correctly indicate neutralization of the formaldehyde must be empirically determined for each particular dye. Generally, this method is more useful for formaldehyde-containing solutions with a relatively small total volume.
In use, the free end of the polystryrene backing serves as a handle for the user. The test strip is held by the handle, and the test pad end is then dipped into the test solution and quickly removed. Alternatively, an aliquot of liquid from the test solution may be removed in a suitable manner and applied to the test strip by pipette, spatula, or swab.
The test strip may be inspected or read for a visual color change by the user after an elapsed time following removal of the test strip from the test solution. This elapsed time or xe2x80x9creaction timexe2x80x9d is a function of the particular indicator dye used. There is usually no need for instrumentation to aid in the inspection. The user may simply note a color change or loss of color, such as those described in the Examples below, to indicate whether formaldehyde has been neutralized in the test solution. Generally, a test strip impregnated with DCIP will turn from blue to white, with the blue color indicating that the none of the DCIP was reduced by sulfite ion and the white color indicating that all of the DCIP was reduced by sulfite ion. Therefore, with respect to tests strips impregnated with DCIP, blue indicates incomplete neutralization of formaldehyde, and white indicates complete neutralization of formaldehyde. A test strip impregnated with thionin will gradually turn from blue to light blue to gray, with the blue color indicating that the thionin was not reduced by sulfite ion, and the gray color indicating that substantially all of the thionin was reduced by sulfite ion. Generally, light blue also indicates that substantially all of the thionin was reduced, depending upon the elapsed reaction time, as shown below in Examples IV and V. Therefore, with respect to test strips impregnated with thionin, blue indicates incomplete neutralization of formaldehyde, light blue may indicate complete neutralization of formaldehyde, and gray indicates complete neutralization of formaldehyde. If the test strip indicates incomplete neutralization following contact with the solution, more neutralizer may then be added to the solution, followed by contacting a second test strip with the solution and observing the color of the second test strip. This process may be repeated until a test strip indicates that the formaldehyde in the solution is neutralized.
Reference color blocks may be provided to aid a user in determining when a color change or loss of color has occurred. For example, a light blue reference color block may be provided with the thionin test strips to show the specific light blue color which indicates complete neutralization of formaldehyde, in order to eliminate ambiguity caused by the varying color interpretations of different users.